1. Field of the Invention
The present invention relates to a method of combining two plate members together and a plate member combining device for carrying out the method.
2. Description of Related Art
A liquid crystal panel to be incorporated into a portable color television set or the like comprises pixels of a TN liquid crystal, a color filter substrate having a surface coated with red, green and blue color dot filters to enable each pixel to display a color, and TFT (thin-film transistor) substrate formed by fabricating a polycrystalline silicon TFT array by vapor deposition on a surface of a glass plate and capable of applying a voltage to each of the pixels. A process of fabricating the liquid crystal panel combines the color filter substrate and the TFT substrate.
Generally, when combining the color filter substrate and the TFT substrate, one of the substrates, for example, the color filter substrate, is held by an equalizing mechanism, such as a spherical seat mechanism, a gimbals mechanism or a gyratory mechanism, so as to be turnable about two axes perpendicularly intersecting each other, and the other plate, for example, the TFT substrate, is held stationarily and, for example, the color filter substrate is moved toward the TFT plate. When the force applied by the TFT substrate to the color filter plate exceeds the static friction force acting between the turning components of the equalizing mechanism, the color filter substrate is turned about the two axes, so that surface of the color filter substrate is aligned with the corresponding surface of the TFT sybstrate and the color filter substrate and the TFT substrate are combined accurately absorbing irregularities in the respective flatnesses and thicknesses of the glass plates of the color filter substrate and the TFT substrates, and a silicon substrate.
Generally, irregularities in flatness and thickness of the glass plates of the color filter substrate and the TFT substrate, and the silicon substrate are very small values on the order of several micrometers and hence it is desirable that the turning members of the equalizing mechanism can be turned by a very small force. However, each turning member of the equalizing mechanism cannot be turned unless a force exceeding the static friction force acting on the turning member is applied to the turning member even if the equalizing mechanism is a gyratory mechanism comprising turning members which can be turned by a comparatively small force. For example, when the turning components of the equalizing mechanism are supported in ball bearings, the respective sphericities of the balls of the ball bearings and the concentricity of the corresponding ball bearings must be improved to the highest possible extent to reduce the force capable of turning the turning components of the equalizing mechanism to the least possible extent. On the other hand, if the static friction force acting on the turning components of the equalizing mechanism is reduced substantially to the dynamic friction force, the turning members holding the plate are not preloaded and hence the plate cannot be stably supported and, consequently, the two plates cannot be combined accurately.
Accordingly, the method of holding the plate has a serious problem that it is very difficult to select parts that produce static friction forces, such as bearings, satisfying two requirements which are contradictory to each other , i.e., preloading the turning members supporting the plate to some extent and enabling the same turning members to be turned by the least possible forces. Furthermore, the variation of the static friction force attributable to abrasion must be continuously monitored and the maintenance of the equalizing mechanism including replacing the abraded component parts must be conducted so that the static friction force may not change.